Multimedia projection systems have become popular for purposes such as conducting sales demonstrations, business meetings, classroom training, and for use in home theaters. In typical operation, multimedia projection systems receive analog video signals from a video unit and convert the video signals to digital information to control one or more digitally driven light valves. Depending on the cost, brightness, and image quality goals of the particular projection systems, the light valves may be of various sizes and resolutions, be transmissive or reflective, and be employed in single or multiple light path configurations. (Hereinafter, projection systems may also be referred to as “projectors.”)
Optimized sets of multimedia projector characteristics have been achieved by employing reflective light valves, the most common types of these reflective light valve optical arrangements are deflected mirror arrays and reflective liquid crystal light valves. Deflected mirror arrays are very efficient reflectors that do not require polarizers for operation. However, they are quite expensive, require off-axis illumination, and often employ unusual optical elements, such as specialized prisms, to compensate for the off-axis light path angles generated.
Reflective liquid crystal light valves are typically fabricated on a silicon substrate and are, therefore, referred to as liquid crystal on silicon (“LCOS”) light valves. They are much less expensive than reflected mirror devices, but require specialized polarizers for operation, which results in significant light transmission losses.
LCOS light valve-based projector architectures employ linear polarized light-sensitive devices for receiving light from a randomly polarized light source, reflecting the light off the light valves, and redirecting the reflected light, depending on its polarization direction or state, either out through a projection lens or back toward the light source. The polarization state of the light is determined by an electronic image pattern applied to the light valve.
There are several different optical architectures for employing LCOS light valves. One variation is a multi-path optical architecture that provides a separate path for each of the primary color (red, blue, and green) lights. The different color lights are routed through a series of polarization beam splitters, filters, and wave plates to a color-specific reflective LCOS light valve. Polychromatic light is optically divided to provide each of the three pathways with its associated color light. A light valve, which is provided in each pathway, is modulated with its respective color data. The individual pathways are then recombined into a converged projected color image. Another variation is a single-path multimedia projector that typically includes a color wheel-based frame-sequential color (“FSC”) optical arrangement. In this arrangement, polychromatic light rays emitted by a light source are directed through the color filter segments of the color wheel. The resulting FSC light travels along a single light path that color timeshares a single light valve.
The multi-path optical architecture generally provides an increased image brightness compared to the single-path architecture. Image brightness is also a function of the amount of collected light from the lamp and the color efficiency, which is generally lower for the single-path architecture. Nevertheless, the single-path architecture is generally preferred because the resulting systems tend to be lighter weight, lower cost, and more compact in size. All of these factors can be further improved if the light produced by the lamp (light source) can be collected efficiently and propagated through the optical components optimized for a low étendue, which enables using reduced-size optical components.
FIGS. 1–3 illustrate these problems in further detail. In particular, FIG. 1 shows a prior art conventional light source 100 used in conjunction with a single-path architecture multimedia projection system. The light source includes an arc lamp 101 mounted at a focus of an elliptical reflector 102. Polychromatic light rays emitted by arc lamp 101 are converged by elliptical reflector 102 to propagate along optical axis 106 through color filter segments of color wheel 103 and optical integrator 104. Color wheel 103 typically includes R, G, B, and light-purplish filter segments. Because the light from arc lamp 101 is typically greenish (deficient in red), the light-purplish (nonwhite) filter segment produces a more accurate white color point and overall color gamut for the multimedia projector. In some multimedia projectors, the color wheel 103 is replaced with other types of color modulators, such as a liquid crystal-based color switcher. For projectors with architecture of a multi-path type, the color modulator is not employed, and polychromatic light propagates directly through the image projection optics. After the FSC light passes through the color wheel 103 (single-path architecture), it passes through an optical integrator before it enters the remaining components of an image projection system 105. The optical integrator 104 is typically an elongated tunnel-type integrator with squared-off flat inlet 104a and outlet 104b ends.
One of the functional purposes of an illumination system is to output a large amount of light energy. However, the emitted light energy is restricted by constraints on the physical dimensions of the light source as well as the amount of light acceptable by downstream optical components. The amount of light that is acceptable to an optical component is a function of its area and the light flux throughput, or étendue. The geometric entity, étendue E, is defined as the product of the transverse sectional area of a light beam and the divergence angle of the beam. Étendue is also referred to as geometric extent.
Referring to FIG. 2, étendue E is a geometric entity that is represented mathematically by Eq. 1:
  E  =            ∫              ∫                              cos            ⁡                          (              Φ              )                                ⁢                      ⅆ            A                    ⁢                      ⅆ            Ω                                =                  A        ⁢                                  ⁢        Ω            =                        A          ⁢                                          ⁢                                    πsin              2                        ⁡                          (              θ              )                                      =                              A            ⁢                                                  ⁢            π                                4            ⁢                                          (                                  f                  /                  #                                )                            2                                          where Ω defines a cone of light 201 diverging through a cross-sectional area A.
Étendue is important because in an optical system it cannot be reduced without a corresponding reduction in light flux. It is of particular importance in the efficient collection of light flux from a light source, such as light source 100, which effectively establishes the lower limit of étendue for the entire optical system.
An illumination system that uses the single on-axis elliptical reflector, as illustrated in FIG. 1, or an on-axis parabolic reflector (not shown), has an intrinsic variation of “magnification-over-angle” that degrades the étendue of the light, thus degrading the output from the illumination source.
FIG. 3 shows one attempted solution to this fundamental limitation of traditional conic reflector arrangements. In particular, light source 301 employs a double-paraboloid reflector 305 having first and second focal points 302 and 307. Light rays are produced by an arc lamp 304 that has its arc located at first focal point 302 of double-paraboloid reflector 305. Arc lamp 304 also includes a mirror coating 303 on its surface facing away from double-paraboloid reflector 305 to reflect light rays 306 back through the arc to join with and further intensify light rays. This light source 301 achieves suitable light-collecting efficiency (greater than about 40%) at a desirable étendue (less than 7 mm2·sr). However, a disadvantage of this light source is that the angles coming off of the second reflection are too steep to be fully utilized by downstream optical components. This stray light reduces the efficiency of the illumination system.
What is still needed, therefore, is an illumination system which achieves a suitable light-collecting efficiency at a small étendue. Such an illumination system would be advantageous in designing a compact, lightweight, and/or low-profile multimedia projection system that achieves a bright and/or high-quality projected image at preferably a relatively low cost.